Hi Savannah, Yes you can use sour cream instead of creme fraiche. But, you have to slightly change the proportion: 250ml of sour cream with 200ml of whole milk instead of 250ml of whole milk with 200ml of creme fraiche.
Because the sour cream contains about 20% of butterfat, so less than the creme fraiche (about 45%). The butterfat is needed to set the salted custard into the quiche. I hope you enjoy the recipe,
Best